This document is the first volume of the 6TiSCH architecture of an IPv6 Multi-Link subnet that is composed of a high speed powered backbone and a number of IEEE802.15.4 TSCH low-power wireless networks attached and synchronized by Backbone Routers. The architecture defines mechanisms to establish and maintain routing and scheduling in a centralized, distributed, or mixed fashion.

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The emergence of wireless technology has enabled a variety of new devices to get interconnected, at a very low marginal cost per device, at any distance ranging from Near Field to interplanetary, and in circumstances where wiring may not be practical, for instance on fast-moving or rotating devices.

At the same time, a new breed of Time Sensitive Networks is being developed to enable traffic that is highly sensitive to jitter, quite sensitive to latency, and with a high degree of operational criticality so that loss should be minimized at all times. Such traffic is not limited to professional Audio/ Video networks, but is also found in command and control operations such as industrial automation and vehicular sensors and actuators. At IEEE802.1, the Audio/Video Task Group [IEEE802.1TSNTG] Time Sensitive Networking (TSN) to address Deterministic Ethernet. The Medium access Control (MAC) of IEEE802.15.4 [IEEE802154] has evolved with the new IEEE802.15.4e TimeSlotted Channel Hopping (TSCH)[I-D.ietf-6tisch-tsch] mode for deterministic industrial-type applications. TSCH was introduced with the IEEE802.15.4e [IEEE802154e] amendment and will be wrapped up in the next revision of the IEEE802.15.4 standard. For all practical purpose, this document is expected to be insensitive to the future versions of the IEEE802.15.4 standard, which is thus referenced undated.

Though at a different time scale, both TSN and TSCH standards provide Deterministic capabilities to the point that a packet that pertains to a certain flow crosses the network from node to node following a very precise schedule, as a train that leaves intermediate stations at precise times along its path. With TSCH, time is formatted into timeSlots, and an individual cell is allocated to unicast or broadcast communication at the MAC level. The time-slotted operation reduces collisions, saves energy, and enables to more closely engineer the network for deterministic properties. The channel hopping aspect is a simple and efficient technique to combat multipath fading and external interference (for example by Wi-Fi emitters).

This document is the first volume of an architecture for an IPv6 Multi-Link subnet that is composed of a high speed powered backbone and a number of IEEE802.15.4 TSCH wireless networks attached and synchronized by backbone routers. Route Computation may be achieved in a centralized fashion by a Path Computation Element (PCE) [PCE], in a distributed fashion using the Routing Protocol for Low Power and Lossy Networks (RPL)[RFC6550], or in a mixed mode. The Backbone Routers may perform proxy IPv6 Neighbor Discovery (ND)[RFC4861] operations over the backbone on behalf of the wireless devices (also called motes), so they can share a same IPv6 subnet and appear to be connected to the same backbone as classical devices. The Backbone Routers may alternatively redistribute the registration in a routing protocol such as OSPF[RFC5340] or BGP[RFC2545], or inject them in a mobility protocol such as MIPv6[RFC6275], NEMO[RFC3963], or LISP[RFC6830].

The 6TiSCH architecture defines four ways a schedule can be managed and TimeSlots can be allocated: Static Scheduling, neighbor-to-neighbor Scheduling, remote monitoring and scheduling management, and Hop-by-hop scheduling. In the case of remote monitoring and scheduling management, TimeSlots and other device resources are managed by an abstract Network Management Entity (NME), which may cooperate with the PCE in order to minimize the interaction with and the load on the constrained device.

The 6TiSCH architecture supports three different forwarding models, G-MPLS Track Forwarding, which switches a frame received at a particular TimeSlot into another TimeStot at Layer-2, 6LoWPAN Fragment Forwarding, which allows to forward individual 6loWPAN fragments along the route set by the first fragment, and classical IPv6 Forwarding, where the node selects a feasible successor at Layer-3 on a per packet basis, based on its routing table.

Some aspects of this architecture derive from existing industrial standards for Process Control such as ISA100.11a [ISA100.11a]and WirelessHART [WirelessHART], by its focus on Deterministic Networking, in particular with the use of the IEEE802.15.4 TSCH MAC and a centralized PCE. This approach leverages the TSCH MAC benefits for high reliability against interference, low-power consumption on deterministic traffic, and its Traffic Engineering capabilities. In such applications, Deterministic Networking applies mainly to control loops and movement detection, but it can also be used for supervisory control flows and management.

An incremental set of industrial requirements is addressed with the addition of an autonomic and distributed routing operation based on RPL. These use-cases include plant setup and decommissioning, as well as monitoring of lots of lesser importance measurements such as corrosion and events. RPL also enables mobile use cases such as mobile workers and cranes, as discussed in [I-D.ietf-roll-rpl-industrial-applicability].

A Backbone Router is included in order to scale the factory plant subnet to address large deployments, with proxy ND and time synchronization over a high speed backbone.

The architecture also applies to building automation that leverage RPL's storing mode to address multipath over a large number of hops, in-vehicle command and control that can be as demanding as industrial applications, commercial automation and asset Tracking with mobile scenarios, home automation and domotics which become more reliable and thus provide a better user experience, and resource management (energy, water, etc.).

Security aspects of the join process by which a device obtains access to the network are discussed in Section 10. With TSCH, devices are time-synchronized at the MAC level. The use of a particular RPL Instance for time synchronization is discussed in Section 7.3. With this mechanism, the time synchronization starts at the RPL root and follows the RPL DODAGs with no timing loop.

The LLN devices communicate over IPv6 [RFC2460] using the 6LoWPAN Header Compression ( 6LoWPAN HC)[RFC6282]. From the perspective of Layer-3, a single LLN interface (typically an IEEE802.15.4-compliant radio) may be seen as a collection of Links with different capabilities for unicast or multicast services. An IPv6 subnet spans over multiple links, effectively forming a Multi-Link subnet. Within that subnet, neighbor devices are discovered with 6LoWPAN Neighbor Discovery[RFC6775] (6LoWPAN ND). RPL[RFC6550] enables routing within the LLN, in the so called Route Over fashion, either in storing (stateful) or non-storing (stateless, with routing headers) mode.

RPL forms Destination Oriented Directed Acyclic Graphs (DODAGs) within Instances of the protocol, each Instance being associated with an Objective Function (OF) to form a routing topology. A particular LLN device, the LLN Border Router (LBR), acts as RPL root, 6LoWPAN HC terminator, and Border Router for the LLN to the outside. The LBR is usually powered. More on RPL Instances can be found in section 3.1 of RPL[RFC6550], in particular "3.1.2. RPL Identifiers" and "3.1.3. Instances, DODAGs, and DODAG Versions".

This architecture expects that a 6LoWPAN node can connect as a leaf to a RPL network, where the leaf support is the minimal functionality to connect as a host to a RPL network without the need to participate to the full routing protocol. The architecture also expects that a 6LoWPAN node that is not aware at all of the RPL protocol may also connect as a host. The derived requirements are listed in [I-D.thubert-6lo-rfc6775-update-reqs].

An extended configuration of the subnet comprises multiple LLNs. The LLNs are interconnected and synchronized over a backbone, that can be wired or wireless. The backbone can be a classical IPv6 network, with Neighbor Discovery operating as defined in [RFC4861] and [RFC4862]. This architecture requires new work to standardize the the registration of 6LoWPAN nodes to the Backbone Routers.

In the extended configuration, a Backbone Router (6BBR) acts as an Energy Aware Default Router (NEAR) as defined in [I-D.chakrabarti-nordmark-6man-efficient-nd]. The 6BBR performs ND proxy operations between the registered devices and the classical ND devices that are located over the backbone. 6TiSCH 6BBRs synchronize with one another over the backbone, so as to ensure that the multiple LLNs that form the IPv6 subnet stay tightly synchronized.

In order to serve nodes that are multiple hops away, an integrated RPL root and 6LBR may be collocated with the 6BBR, or attached to the 6BBR in which case they would perform the registration on behalf of the remote LLN nodes - they proxy the efficient ND registration over the LLN in order for the 6BBR to perform proxy ND operations over the backbone.

If the Backbone is Deterministic (such as defined by the Time Sensitive Networking WG at IEEE), then the Backbone Router ensures that the end-to-end deterministic behavior is maintained between the LLN and the backbone. The DetNet Architecture[I-D.finn-detnet-architecture] studies Layer-3 aspects of Deterministic Networks, and covers networks that span multiple Layer-2 domains.

In order to control the complexity and the size of the 6TiSCH work, the architecture and the associated IETF work are staged in volumes. This document covers the first stage of the work, as specified by the WG charter. If the work continues as expected, further volumes will complete this piece and provide the full coverage of IPv6 over TSCH.

The main architectural blocks are represented below to help detail what is covered and what is not yet covered from the global 6TiSCH architecture by this initial volume:

RPL is the routing protocol of choice for LLNs. So far, there was no identified need to define a 6TiSCH specific Objective Function. The Minimal 6TiSCH Configuration [I-D.ietf-6tisch-minimal] describes the operation of RPL over a static schedule used in a slotted aloha fashion, whereby all active slots may be used for emission or reception of both unicast and multicast frames.

The architecture of the operation of RPL over a dynamic schedule is deferred to a subsequent volume of the architecture.

The work on centralized track computation is deferred to a subsequent volume of the architecture. The Path Computation Element (PCE) is certainly the core component of that architecture. Around the PCE, a protocol such as an extension to a TEAS [TEAS] protocol (maybe running over CoAP as illustrated) will be required to expose the device capabilities and the network peers to the PCE, and a protocol such as a lightweight PCEP or an adaptation of CCAMP [CCAMP] G-MPLS formats and procedures will be used to publish the tracks, computed by the PCE, to the devices (maybe in a fashion similar to RSVP-TE).

The selection of an authentication, an authorization and a Transport layer security protocols are out of scope for this volume.

Similarly, the Protocol for Carrying Authentication for Network access (PANA)[RFC5191] is represented as an example of a protocol that could be leveraged to secure the join process, as a Layer-3 alternate to IEEE802.1x/EAP. Work resulting from [ACE] could be considered as well. Regardless, the security model must ensure that, prior to a join process, packets from a untrusted device are controlled in volume and in reachability. An overview of the security aspects of the join process can be found in Section 10. Related contributions are presented in Appendix A.

The 6TiSCH Operation sublayer (6top)[I-D.wang-6tisch-6top-sublayer] is an Logical Link Control (LLC) or a portion thereof that provides the abstraction of an IP link over a TSCH MAC. The work on the operations of that layer, in particular related to dynamic scheduling, is only introduced here, and should be detailed further in a subsequent volume of the architecture.

At the time of this writing, the components and protocols that are required to implement this stage of architecture are not fully available from the IETF. In particular, the requirements on an evolution of 6LoWPAN Neighbor Discovery that are needed to implement the Backbone Router as covered by this stage of the architecture are detailed in [I-D.thubert-6lo-rfc6775-update-reqs].

The 6TiSCH Architecture applies the concepts of Deterministic Networking on a Layer-3 network. The 6TiSCH Architecture should inherit from DetNet[I-D.finn-detnet-architecture] work and thus depends on it. In turn, DetNet is expected to integrate and maintain consistency with the work that has taken place and is continuing at IEEE802.1TSN and AVnu.

The current charter positions 6TiSCH on IEEE802.15.4 only. Though most of the design should be portable on other link types, 6TiSCH has a strong dependency on IEEE802.15.4 and its evolution. A new version of the IEEE802.15.4 standard is expected in 2015. That version should integrate TSCH as well as other amendments and fixes into the main specification. The impact on this Architecture should be minimal to non-existent, but deeper work such as 6top and security may be impacted. A 6TiSCH Interest Group was formed at IEEE to maintain the synchronization and help foster work at the IEEE should 6TiSCH demand it.

ISA100 [ISA100] Common Network Management (CNM) is another external work of interest for 6TiSCH. The group, referred to as ISA100.20, defines a Common Network Management framework that should enable the management of resources that are controlled by heterogeneous protocols such as ISA100.11a [ISA100.11a], WirelessHART [WirelessHART], and 6TiSCH. Interestingly, the establishment of 6TiSCH Deterministic paths, called tracks, are also in scope, and ISA100.20 is working on requirements for DetNet.

The architecture expects that a 6LoWPAN node that is not aware at all of the RPL protocol may still connect as a host. It suggests to extend 6LoWPAN ND [RFC6775] to carry the sequence number that is needed by RPL to track the movements of the device, and optionally some abstract information about the RPL instance (topology) that the device will be reachable over.

In this design, the root of the RPL network is integrated with the 6LoWPAN ND 6LBR, but it is logically separated from the Backbone Router (6BBR) that is used to connect the RPL topology to the backbone. This way, the root has all information from 6LoWPAN ND and RPL about the LLN devices attached to it.

This architecture also expects that the root of the RPL network (proxy-)registers the LLN devices on their behalf to the 6BBR, for whatever operation the 6BBR performs on the backbone, such as ND proxy, or redistribution in a routing protocol. It suggests to use an extension of the mixed mode of Efficient ND [I-D.chakrabarti-nordmark-6man-efficient-nd] for the registration as described in [I-D.thubert-6lowpan-backbone-router].

It results that, as illustrated in Figure 4, the periodic signaling would start at the leaf node with 6LoWPAN ND, then would be carried over RPL to the RPL root, and then with Efficient-ND to the 6BBR. Efficient ND being an adaptation of 6LoWPAN ND, it makes sense to keep those two homogeneous in the way they use the source and the target addresses in the Neighbor Solicitation (NS) messages for registration, as well as in the options that they use for that process.

As the network builds up, a node should start as a leaf to join the RPL network, and may later turn into both a RPL-capable router and a 6LR, so as to accept leaf nodes to recursively join the network.

RPL needs a set of information in order to advertise a leaf node through a DAO message and establish reachability.

At the bare minimum the leaf device must provide a sequence number that matches the RPL specification in section 7. Section 4.1 of [I-D.chakrabarti-nordmark-6man-efficient-nd], on the Address Registration Option (ARO), already incorporates that addition with a new field in the option called the Transaction ID.

If for some reason the node is aware of RPL topologies, then providing the RPL InstanceID for the instances to which the node wishes to participate would be a welcome addition. In the absence of such information, the RPL router must infer the proper instanceID from external rules and policies.

On the backbone, the InstanceID is expected to be mapped onto a an overlay that matches the instanceID, for instance a VLANID.

Registration to the 6LBR through DAR/DAC messages [RFC6775] may percolate slowly through an LLN mesh, and it might happen that in the meantime, the 6LoWPAN node moves and registers somewhere else. Both RPL and 6LoWPAN ND lack the capability to indicate that the same node is registered elsewhere, so as to invalidate states down the deprecated path.

In its current expression and functionality, 6LoWPAN ND considers that the registration is used for the purpose of DAD only as opposed to that of achieving reachability, and as long as the same node registers the IPv6 address, the protocol is functional. In order to act as a RPL leaf registration protocol and achieve reachability, the device must use the same TID for all its concurrent registrations, and registrations with a past TID should be declined. The state for an obsolete registration in the 6LR, as well as the RPL routers on the way, should be invalidated. This can only be achieved with the addition of a new Status in the DAC message, and a new error/clean-up flow in RPL.

The 6BBR provides the capability to defend an address that is owned by a 6LoWPAN Node, and attract packets to that address, whether it is done by proxying ND over a MultiLink Subnet, redistributing the address in a routing protocol or advertising it through an alternate proxy registration such as the Locator/ID Separation Protocol[RFC6830] (LISP) or Mobility Support in IPv6[RFC6275] (MIPv6). In a LLN, it makes sense to piggyback the request to proxy/defend an address with its registration.

In their current incarnations, both 6LoWPAN ND and Efficient ND expect that the address being registered is the source of the NS(ARO) message and thus impose that a Source Link-Layer Address (SLLA) option be present in the message. In a mesh scenario where the 6LBR is physically separated from the 6LoWPAN Node, the 6LBR does not own the address being registered. This suggests that [I-D.chakrabarti-nordmark-6man-efficient-nd] should evolve to register the Target of the NS message as opposed to the Source Address. From another perspective, it may happen, in the use case of a Star topology, that the 6LR, 6LBR and 6BBR are effectively collapsed and should support 6LoWPAN ND clients. The convergence of efficient ND and 6LoWPAN ND into a single protocol is thus highly desirable.

In any case, as long as the DAD process is not complete for the address used as source of the packet, it is against the current practice to advertise the SLLA, since this may corrupt the ND cache of the destination node, as discussed in the Optimistic DAD specification[RFC4429] with regards to the TENTATIVE state.

This may look like a chicken and an egg problem, but in fact 6LoWPAN ND acknowledges that the Link-Local Address that is based on an EUI-64 address of a LLN node may be autoconfigured without the need for DAD. It results that a node could use that Address as source, with an SLLA option in the message if required, to register any other addresses, either Global or Unique-Local Addresses, which would be indicated in the Target.

The suggested change is to register the target of the NS message, and use Target Link-Layer Address (TLLA) in the NS as opposed to the SLLA in order to install a Neighbor Cache Entry. This would apply to both Efficient ND and 6LoWPAN ND in a very same manner, with the caveat that depending on the nature of the link between the 6LBR and the 6BBR, the 6LBR may resort to classical ND or DHCPv6 to obtain the address that it uses to source the NS registration messages, whether for itself or on behalf of LLN nodes.

6LoWPAN ND is unclear on how the 6LBR is discovered, and how the liveliness of the 6LBR is asserted over time. On the other hand, the discovery and liveliness of the RPL root are obtained through the RPL protocol.

When 6LoWPAN ND is coupled with RPL, the 6LBR and RPL root functionalities are co-located in order that the address of the 6LBR be indicated by RPL DIO messages and to associate the unique ID from the DAR/DAC exchange with the state that is maintained by RPL. The DAR/DAC exchange becomes a preamble to the DAO messages that are used from then on to reconfirm the registration, thus eliminating a duplication of functionality between DAO and DAR messages.

A typical attack against IPv6 ND is address spoofing, whereby a rogue node claims the IPv6 Address of another node in and hijacks its traffic. The threats against IPv6 ND as described in SEcure Neighbor Discovery (SEND)[RFC3971] are applicable to 6LoPWAN ND as well, but the solution can not work as the route over network does not permit direct peer to peer communication.

Additionally SEND requires considerably enlarged ND messages to carry cryptographic material, and requires that each protected address is generated cryptographically, which implies the computation of a different key for each Cryptographically Generated Address (CGA). SEND as defined in [RFC3971] is thus largely unsuitable for application in a LLN.

With 6LoWPAN ND, as illustrated in Figure 4, it is possible to leverage the registration state in the 6LBR, which may store additional security information for later proof of ownership. If this information proves the ownership independently of the address itself, then a single proof may be used to protect multiple addresses.

Once an Address is registered, the 6LBR maintains a state for that Address and is in position to bind securely the first registration with the Node that placed it, whether the Address is CGA or not. It should thus be possible to protect the ownership of all the addresses of a 6LoWPAN Node with a single key, and there should not be a need to carry the cryptographic material more than once to the 6LBR.

The energy constraint is usually a foremost factor, and attention should be paid to minimize the burden on the CPU. Hardware-assisted support of variants of the Counter with CBC-MAC[RFC3610] (CCM) authenticated encryption block cipher mode such as CCM* are common in LowPower ship-set implementations, and 6LoWPAN ND security mechanism should be capable to reuse them when applicable.

Finally, the code footprint in the device being also an issue, the capability to reuse not only hardware-assist mechanisms but also software across layers has to be considered. For instance, if code has to be present for upper-layer operations, e.g AES-CCM Cipher Suites for Transport Layer Security (TLS)[RFC6655], then the capability to reuse that code should be considered.

6top is a logical link control sitting between the IP layer and the TSCH MAC layer, which provides the link abstraction that is required for IP operations. The 6top operations are specified in [I-D.wang-6tisch-6top-sublayer]. In particular, 6top provides a management interface that enables an external management entity to schedule cells and slotFrames, and allows the addition of complementary functionality, for instance to support a dynamic schedule management based on observed resource usage as discussed in Section 8.1.2.

The 6top data model and management interfaces are further discussed in Section 8.1.3.

The architecture defines "soft" cells and "hard" cells. "Hard" cells are owned and managed by an separate scheduling entity (e.g. a PCE) that specifies the slotOffset/channelOffset of the cells to be added/moved/deleted, in which case 6top can only act as instructed, and may not move hard cells in the TSCH schedule on its own.

6top contains a monitoring process which monitors the performance of cells, and can move a cell in the TSCH schedule when it performs poorly. This is only applicable to cells which are marked as "soft". To reserve a soft cell, the higher layer does not indicate the exact slotOffset/channelOffset of the cell to add, but rather the resulting bandwidth and QoS requirements. When the monitoring process triggers a cell reallocation, the two neighbor devices communicating over this cell negotiate its new position in the TSCH schedule.

Most OFs require metrics about reachability, such as the ETX. 6top creates and maintains an abstract neighbor table, and this state may be leveraged to feed an OF and/or store OF information as well. In particular, 6top creates and maintains an abstract neighbor table. A neighbor table entry contains a set of statistics with respect to that specific neighbor including the time when the last packet has been received from that neighbor, a set of cell quality metrics (e.g. RSSI or LQI), the number of packets sent to the neighbor or the number of packets received from it. This information can be obtained through 6top management APIs as detailed in the 6top sublayer specification [I-D.wang-6tisch-6top-sublayer] and used for instance to compute a Rank Increment that will determine the selection of the preferred parent.

6top provides statistics about the underlying layer so the OF can be tuned to the nature of the TSCH MAC layer. 6top also enables the RPL OF to influence the MAC behaviour, for instance by configuring the periodicity of IEEE802.15.4 Extended Beacons (EB's). By augmenting the EB periodicity, it is possible to change the network dynamics so as to improve the support of devices that may change their point of attachment in the 6TiSCH network.

Some RPL control messages, such as the DODAG Information Object (DIO) are ICMPv6 messages that are broadcast to all neighbor nodes. With 6TiSCH, the broadcast channel requirement is addressed by 6top by configuring TSCH to provide a broadcast channel, as opposed to, for instance, piggybacking the DIO messages in Enhance Beacons. Consideration was given towards finding a way to embed the Route Advertisements and the RPL DIO messages (both of which are multicast) into the IEEE802.15.4 Enhanced Beacons. It was determined that this produced undue timer coupling among layers, that the resulting packet size was potentially too large, and required it is not yet clear that there is any need for Enhanced Beacons in a production network.

Nodes in a TSCH network must be time synchronized. A node keeps synchronized to its time source neighbor through a combination of frame-based and acknowledgment-based synchronization. In order to maximize battery life and network throughput, it is advisable that RPL ICMP discovery and maintenance traffic (governed by the trickle timer) be somehow coordinated with the transmission of time synchronization packets (especially with enhanced beacons). This could be achieved through an interaction of the 6top sublayer and the RPL objective Function, or could be controlled by a management entity.

Time distribution requires a loop-less structure. Nodes taken in a synchronization loop will rapidly desynchronize from the network and become isolated. It is expected that a RPL DAG with a dedicated global Instance is deployed for the purpose of time synchronization. That Instance is referred to as the Time Synchronization Global Instance (TSGI). The TSGI can be operated in either of the 3 modes that are detailed in section 3.1.3 of RPL[RFC6550], "Instances, DODAGs, and DODAG Versions". Multiple uncoordinated DODAGs with independent roots may be used if all the roots share a common time source such as the Global Positioning System (GPS). In the absence of a common time source, the TSGI should form a single DODAG with a virtual root. A backbone network is then used to synchronize and coordinate RPL operations between the backbone routers that act as sinks for the LLN. Optionally, RPL's periodic operations may be used to transport the network synchronization. This may mean that 6top would need to trigger (override) the trickle timer if no other traffic has occurred for such a time that nodes may get out of synchronization.

A node that has not joined the TSGI advertises a MAC level Join Priority of 0xFF to notify its neighbors that is not capable of serving as time parent. A node that has joined the TSGI advertises a MAC level Join Priority set to its DAGRank() in that Instance, where DAGRank() is the operation specified in section 3.5.1 of [RFC6550], "Rank Comparison".

A root is configured or obtains by some external means the knowledge of the RPLInstanceID for the TSGI. The root advertises its DagRank in the TSGI, that must be less than 0xFF, as its Join Priority (JP) in its IEEE802.15.4 Extended Beacons (EB). We'll note that the JP is now specified between 0 and 0x3F leaving 2 bits in the octet unused in the IEEE802.15.4e specification. After consultation with IEEE authors, it was asserted that 6TiSCH can make a full use of the octet to carry an integer value up to 0xFF.

A node that reads a Join Priority of less than 0xFF should join the neighbor with the lesser Join Priority and use it as time parent. If the node is configured to serve as time parent, then the node should join the TSGI, obtain a Rank in that Instance and start advertising its own DagRank in the TSGI as its Join Priority in its EBs.

6TiSCH enables in essence the capability to use IPv6 over a MAC layer that enables to schedule some of the transmissions. In order to ensure that the medium is free of contending packets when time arrives for a scheduled transmission, a window of time is defined around the scheduled transmission time where the medium must be free of contending energy.

One simple way to obtain such a window is to format time and frequencies in cells of transmission of equal duration. This is the method that is adopted in IEEE802.15.4 TSCH as well as the Long Term Evolution (LTE) of cellular networks.

In order to describe that formatting of time and frequencies, the 6TiSCH architecture defines a global concept that is called a Channel Distribution and Usage (CDU) matrix; a CDU matrix is a matrix of cells with an height equal to the number of available channels (indexed by ChannelOffsets) and a width (in timeSlots) that is the period of the network scheduling operation (indexed by slotOffsets) for that CDU matrix. The size of a cell is a timeSlot duration, and values of 10 to 15 milliseconds are typical in 802.15.4 TSCH to accommodate for the transmission of a frame and an ack, including the security validation on the receive side which may take up to a few milliseconds on some device architecture.

A CDU matrix iterates over and over with a pseudo-random rotation from an epoch time. In a given network, there might be multiple CDU matrices that operate with different width, so they have different durations and represent different periodic operations. It is recommended that all CDU matrices in a 6TiSCH domain operate with the same cell duration and are aligned, so as to reduce the chances of interferences from slotted-aloha operations. The knowledge of the CDU matrices is shared between all the nodes and used in particular to define slotFrames.

A slotFrame is a MAC-level abstraction that is common to all nodes and contains a series of timeSlots of equal length and precedence. It is characterized by a slotFrame_ID, and a slotFrame_size. A slotFrame aligns to a CDU matrix for its parameters, such as number and duration of timeSlots.

Multiple slotFrames can coexist in a node schedule, i.e., a node can have multiple activities scheduled in different slotFrames, based on the precedence of the 6TiSCH topologies. The slotFrames may be aligned to different CDU matrices and thus have different width. There is typically one slotFrame for scheduled traffic that has the highest precedence and one or more slotFrame(s) for RPL traffic. The timeSlots in the slotFrame are indexed by the SlotOffset; the first cell is at SlotOffset 0.

When a packet is received from a higher layer for transmission, 6top inserts that packet in the outgoing queue which matches the packet best (Differentiated Services [RFC2474] can therefore be used). At each scheduled transmit slot, 6top looks for the frame in all the outgoing queues that best matches the cells. If a frame is found, it is given to the TSCH MAC for transmission.

6TiSCH expects a high degree of scalability together with a distributed routing functionality based on RPL. To achieve this goal, the spectrum must be allocated in a way that allows for spatial reuse between zones that will not interfere with one another. In a large and spatially distributed network, a 6TiSCH node is often in a good position to determine usage of spectrum in its vicinity.

Use cases for distributed routing are often associated with a statistical distribution of best-effort traffic with variable needs for bandwidth on each individual link. With 6TiSCH, the link abstraction is implemented as a bundle of cells; the size of a bundle is optimal when both the energy wasted idle listening and the packet drops due to congestion loss are minimized. This can be maintained if the number of cells in a bundle is adapted dynamically, and with enough reactivity, to match the variations of best-effort traffic. In turn, the agility to fulfill the needs for additional cells improves when the number of interactions with other devices and the protocol latencies are minimized.

6TiSCH limits that interaction to RPL parents that will only negotiate with other RPL parents, and performs that negotiation by groups of cells as opposed to individual cells. The 6TiSCH architecture allows RPL parents to adjust dynamically, and independently from the PCE, the amount of bandwidth that is used to communicate between themselves and their children, in both directions; to that effect, an allocation mechanism enables a RPL parent to obtain the exclusive use of a portion of a CDU matrix within its interference domain. Note that a PCE is expected to have precedence in the allocation, so that a RPL parent would only be able to obtain portions that are not in-use by the PCE.

The 6TiSCH architecture introduces the concept of chunks [I-D.ietf-6tisch-terminology]) to operate such spectrum distribution for a whole group of cells at a time. The CDU matrix is formatted into a set of chunks, each of them identified uniquely by a chunk-ID. The knowledge of this formatting is shared between all the nodes in a 6TiSCH network. 6TiSCH also defines the process of chunk ownership appropriation whereby a RPL parent discovers a chunk that is not used in its interference domain (e.g lack of energy detected in reference cells in that chunk); then claims the chunk, and then defends it in case another RPL parent would attempt to appropriate it while it is in use. The chunk is the basic unit of ownership that is used in that process.

As a result of the process of chunk ownership appropriation, the RPL parent has exclusive authority to decide which cell in the appropriated chunk can be used by which node in its interference domain. In other words, it is implicitly delegated the right to manage the portion of the CDU matrix that is represented by the chunk. The RPL parent may thus orchestrate which transmissions occur in any of the cells in the chunk, by allocating cells from the chunk to any form of communication (unicast, multicast) in any direction between itself and its children. Initially, those cells are added to the heap of free cells, then dynamically placed into existing bundles, in new bundles, or allocated opportunistically for one transmission.

The appropriation of a chunk can also be requested explicitly by the PCE to any node. In that case, the node still may need to perform the appropriation process to validate that no other node has claimed that chunk already. After a successful appropriation, the PCE owns the cells in that chunk, and may use them as hard cells to set up tracks.

[I-D.ietf-6tisch-terminology] defines the terms of Communication Paradigms and Interaction Models, which can be placed in parallel to the Information Models and Data Models that are defined in [RFC3444].

A Communication Paradigms would be an abstract view of a protocol exchange, and would come with an Information Model for the information that is being exchanged. In contrast, an Interaction Models would be more refined and could point on standard operation such as a Representational state transfer (REST) "GET" operation and would match a Data Model for the data that is provided over the protocol exchange.

section 2.1.3 of [I-D.ietf-roll-rpl-industrial-applicability] and next sections discuss application-layer paradigms, such as Source-sink (SS) that is a Multipeer to Multipeer (MP2MP) model primarily used for alarms and alerts, Publish-subscribe (PS, or pub/sub) that is typically used for sensor data, as well as Peer-to-peer (P2P) and Peer-to-multipeer (P2MP) communications. Additional considerations on Duocast and its N-cast generalization are also provided. Those paradigms are frequently used in industrial automation, which is a major use case for IEEE802.15.4 TSCH wireless networks with [ISA100.11a] and [WirelessHART], that provides a wireless access to [HART] applications and devices.

This specification focuses on Communication Paradigms and Interaction Models for packet forwarding and TSCH resources (cells) management. Management mechanisms for the TSCH schedule at Link-layer (one-hop), Network-layer (multithop along a track), and Application-layer (remote control) are discussed in Section 8.1. Link-layer frame forwarding interactions are discussed in Section 8.2, and Network-layer Packet routing is addressed in Section 8.3.

6TiSCH uses 4 paradigms to manage the TSCH schedule of the LLN nodes: Static Scheduling, neighbor-to-neighbor Scheduling, remote monitoring and scheduling management, and Hop-by-hop scheduling. Multiple mechanisms are defined that implement the associated Interaction Models, and can be combined and used in the same LLN. Which mechanism(s) to use depends on application requirements.

In the simplest instantiation of a 6TiSCH network, a common fixed schedule may be shared by all nodes in the network. Cells are shared, and nodes contend for slot access in a slotted aloha manner.

A static TSCH schedule can be used to bootstrap a network, as an initial phase during implementation, or as a fall-back mechanism in case of network malfunction. This schedule can be preconfigured or learnt by a node when joining the network. Regardless, the schedule remains unchanged after the node has joined a network. The Routing Protocol for LLNs (RPL) is used on the resulting network. This "minimal" scheduling mechanism that implements this paradigm is detailed in [I-D.ietf-6tisch-minimal].

In the simplest instantiation of a 6TiSCH network described in Section 8.1.1, nodes may expect a packet at any cell in the schedule and will waste energy idle listening. In a more complex instantiation of a 6TiSCH network, a matching portion of the schedule is established between peers to reflect the observed amount of transmissions between those nodes. The aggregation of the cells between a node and a peer forms a bundle that the 6top layer uses to implement the abstraction of a link for IP. The bandwidth on that link is proportional to the number of cells in the bundle.

If the size of a bundle is configured to fit an average amount of bandwidth, peak traffic is dropped. If the size is configured to allow for peak emissions, energy is be wasted idle listening.

In the most efficient instantiation of a 6TiSCH network, the size of the bundles that implement the links may be changed dynamically in order to adapt to the need of end-to-end flows routed by RPL. An optional On-The-Fly (OTF) component may be used to monitor bandwidth usage and perform requests for dynamic allocation by the 6top sublayer. The OTF component is not part of the 6top sublayer. It may be collocated on the same device or may be partially or fully offloaded to an external system.

The 6top sublayer [I-D.wang-6tisch-6top-sublayer] defines a protocol for neighbor nodes to reserve soft cells to one another. Because this reservation is done without global knowledge of the schedule of nodes in the LLN, scheduling collisions are possible. 6top defines a monitoring process which continuously tracks the packet delivery ratio of soft cells. It uses these statistics to trigger the reallocation of a soft cell in the schedule, using a negotiation protocol between the neighbors nodes communicating over that cell.

Monitoring and relocation is done in the 6top layer. For the upper layer, the connection between two neighbor nodes appears as an number of cells. Depending on traffic requirements, the upper layer can request 6top to add or delete a number of cells scheduled to a particular neighbor, without being responsible for choosing the exact slotOffset/channelOffset of those cells.

The 6top interface document [I-D.ietf-6tisch-6top-interface] specifies the generic data model that can be used to monitor and manage resources of the 6top sublayer. Abstract methods are suggested for use by a management entity in the device. The data model also enables remote control operations on the 6top sublayer.

The capability to interact with the node 6top sublayer from multiple hops away can be leveraged for monitoring, scheduling, or a combination of thereof. The architecture supports variations on the deployment model, and focuses on the flows rather than whether there is a proxy or a translation operation en-route.

[I-D.ietf-6tisch-coap] defines a basic set CoAP resources and associated RESTful access methods (GET/PUT/POST/DELETE). The payload (body) of the CoAP messages is encoded using the CBOR format. The draft also defines the concept of "profiles" to allow for future or specific extensions, as well as a mechanism for a CoAP client to discover the profiles installed on a node.

The entity issuing the CoAP requests can be a central scheduling entity (e.g. a PCE), a node multiple hops away with the authority to modify the TSCH schedule (e.g. the head of a local cluster), or a external device monitoring the overall state of the network (e.g. NME). It is also possible that a mapping entity on the backbone transforms a non-CoAP protocol such as PCEP into the RESTful interfaces that the 6TiSCH devices support.

A node can reserve a track to a destination node multiple hops away by installing soft cells at each intermediate node. This forms a track of soft cells. It is the responsibility of the 6top sublayer of each node on the track to monitor these soft cells and trigger relocation when needed.

This hop-by-hop reservation mechanism is expected to be similar in essence to [RFC3209] and/or [RFC4080]/[RFC5974]. The protocol for a node to trigger hop-by-hop scheduling is not yet defined.

By forwarding, this specification means the per-packet operation that allows to deliver a packet to a next hop or an upper layer in this node. Forwarding is based on pre-existing state that was installed as a result of a routing computation Section 8.3. 6TiSCH supports three different forwarding model, G-MPLS Track Forwarding (TF), 6LoWPAN Fragment Forwarding (FF) and IPv6 Forwarding (6F).

A Track is a unidirectional path between a source and a destination. In a Track cell, the normal operation of IEEE802.15.4 Automatic Repeat-reQuest (ARQ) usually happens, though the acknowledgment may be omitted in some cases, for instance if there is no scheduled cell for a retry.

Track Forwarding is the simplest and fastest. A bundle of cells set to receive (RX-cells) is uniquely paired to a bundle of cells that are set to transmit (TX-cells), representing a layer-2 forwarding state that can be used regardless of the network layer protocol. This model can effectively be seen as a Generalized Multi-protocol Label Switching (G-MPLS) operation in that the information used to switch a frame is not an explicit label, but rather related to other properties of the way the packet was received, a particular cell in the case of 6TiSCH. As a result, as long as the TSCH MAC (and Layer-2 security) accepts a frame, that frame can be switched regardless of the protocol, whether this is an IPv6 packet, a 6LoWPAN fragment, or a frame from an alternate protocol such as WirelessHART or ISA100.11a.

A data frame that is forwarded along a Track normally has a destination MAC address that is set to broadcast - or a multicast address depending on MAC support. This way, the MAC layer in the intermediate nodes accepts the incoming frame and 6top switches it without incurring a change in the MAC header. In the case of IEEE802.15.4, this means effectively broadcast, so that along the Track the short address for the destination of the frame is set to 0xFFFF.

A Track is thus formed end-to-end as a succession of paired bundles, a receive bundle from the previous hop and a transmit bundle to the next hop along the Track, and a cell in such a bundle belongs to at most one Track. For a given iteration of the device schedule, the effective channel of the cell is obtained by adding a pseudo-random number to the channelOffset of the cell, which results in a rotation of the frequency that used for transmission. The bundles may be computed so as to accommodate both variable rates and retransmissions, so they might not be fully used at a given iteration of the schedule. The 6TiSCH architecture provides additional means to avoid waste of cells as well as overflows in the transmit bundle, as follows:

In one hand, a TX-cell that is not needed for the current iteration may be reused opportunistically on a per-hop basis for routed packets. When all of the frame that were received for a given Track are effectively transmitted, any available TX-cell for that Track can be reused for upper layer traffic for which the next-hop router matches the next hop along the Track. In that case, the cell that is being used is effectively a TX-cell from the Track, but the short address for the destination is that of the next-hop router. It results that a frame that is received in a RX-cell of a Track with a destination MAC address set to this node as opposed to broadcast must be extracted from the Track and delivered to the upper layer (a frame with an unrecognized MAC address is dropped at the lower MAC layer and thus is not received at the 6top sublayer).

On the other hand, it might happen that there are not enough TX-cells in the transmit bundle to accommodate the Track traffic, for instance if more retransmissions are needed than provisioned. In that case, the frame can be placed for transmission in the bundle that is used for layer-3 traffic towards the next hop along the track as long as it can be routed by the upper layer, that is, typically, if the frame transports an IPv6 packet. The MAC address should be set to the next-hop MAC address to avoid confusion. It results that a frame that is received over a layer-3 bundle may be in fact associated to a Track. In a classical IP link such as an Ethernet, off-track traffic is typically in excess over reservation to be routed along the non-reserved path based on its QoS setting. But with 6TiSCH, since the use of the layer-3 bundle may be due to transmission failures, it makes sense for the receiver to recognize a frame that should be re-tracked, and to place it back on the appropriate bundle if possible. A frame should be re-tracked if the Per-Hop-Behavior group indicated in the Differentiated Services Field in the IPv6 header is set to Deterministic Forwarding, as discussed in Section 8.3.1. A frame is re-tracked by scheduling it for transmission over the transmit bundle associated to the Track, with the destination MAC address set to broadcast.

In transport mode, the Protocol Data Unit (PDU) is associated with flow-dependant meta-data that refers uniquely to the Track, so the 6top sublayer can place the frame in the appropriate cell without ambiguity. In the case of IPv6 traffic, this flow identification is transported in the Flow Label of the IPv6 header. Associated with the source IPv6 address, the Flow Label forms a globally unique identifier for that particular Track that is validated at egress before restoring the destination MAC address (DMAC) and punting to the upper layer.

In tunnel mode, the frames originate from an arbitrary protocol over a compatible MAC that may or may not be synchronized with the 6TiSCH network. An example of this would be a router with a dual radio that is capable of receiving and sending WirelessHART or ISA100.11a frames with the second radio, by presenting itself as an access Point or a Backbone Router, respectively.

In that mode, some entity (e.g. PCE) can coordinate with a WirelessHART Network Manager or an ISA100.11a System Manager to specify the flows that are to be transported transparently over the Track.

In that case, the flow information that identifies the Track at the ingress 6TiSCH router is derived from the RX-cell. The dmac is set to this node but the flow information indicates that the frame must be tunneled over a particular Track so the frame is not passed to the upper layer. Instead, the dmac is forced to broadcast and the frame is passed to the 6top sublayer for switching.

At the egress 6TiSCH router, the reverse operation occurs. Based on metadata associated to the Track, the frame is passed to the appropriate link layer with the destination MAC restored.

Metadata coming with the Track configuration is expected to provide the destination MAC address of the egress endpoint as well as the tunnel mode and specific data depending on the mode, for instance a service access point for frame delivery at egress. If the tunnel egress point does not have a MAC address that matches the configuration, the Track installation fails.

In transport mode, if the final layer-3 destination is the tunnel termination, then it is possible that the IPv6 address of the destination is compressed at the 6LoWPAN sublayer based on the MAC address. It is thus mandatory at the ingress point to validate that the MAC address that was used at the 6LoWPAN sublayer for compression matches that of the tunnel egress point. For that reason, the node that injects a packet on a Track checks that the destination is effectively that of the tunnel egress point before it overwrites it to broadcast. The 6top sublayer at the tunnel egress point reverts that operation to the MAC address obtained from the tunnel metadata.

Considering that 6LoWPAN packets can be as large as 1280 bytes (the IPv6 MTU), and that the non-storing mode of RPL implies Source Routing that requires space for routing headers, and that a IEEE802.15.4 frame with security may carry in the order of 80 bytes of effective payload, an IPv6 packet might be fragmented into more than 16 fragments at the 6LoWPAN sublayer.

This level of fragmentation is much higher than that traditionally experienced over the Internet with IPv4 fragments, where fragmentation is already known as harmful.

In the case to a multihop route within a 6TiSCH network, Hop-by-Hop recomposition occurs at each hop in order to reform the packet and route it. This creates additional latency and forces intermediate nodes to store a portion of a packet for an undetermined time, thus impacting critical resources such as memory and battery.

[I-D.thubert-roll-forwarding-frags] describes a mechanism whereby the datagram tag in the 6LoWPAN Fragment is used as a label for switching at the 6LoWPAN sublayer. The draft allows for a degree of flow control based on an Explicit Congestion Notification, as well as end-to-end individual fragment recovery.

In that model, the first fragment is routed based on the IPv6 header that is present in that fragment. The 6LoWPAN sublayer learns the next hop selection, generates a new datagram tag for transmission to the next hop, and stores that information indexed by the incoming MAC address and datagram tag. The next fragments are then switched based on that stored state.

A bitmap and an ECN echo in the end-to-end acknowledgment enable the source to resend the missing fragments selectively. The first fragment may be resent to carve a new path in case of a path failure. The ECN echo set indicates that the number of outstanding fragments should be reduced.

6TiSCH supports a mixed model of centralized routes and distributed routes. Centralized routes can for example be computed by a entity such as a PCE. Distributed routes are computed by RPL.

Both methods may inject routes in the Routing Tables of the 6TiSCH routers. In either case, each route is associated with a 6TiSCH topology that can be a RPL Instance topology or a track. The 6TiSCH topology is indexed by a Instance ID, in a format that reuses the RPLInstanceID as defined in RPL[RFC6550].

Both RPL and PCE rely on shared sources such as policies to define Global and Local RPLInstanceIDs that can be used by either method. It is possible for centralized and distributed routing to share a same topology. Generally they will operate in different slotFrames, and centralized routes will be used for scheduled traffic and will have precedence over distributed routes in case of conflict between the slotFrames.

All packets inside a 6TiSCH domain must carry the Instance ID that identifies the 6TiSCH topology that is to be used for routing and forwarding that packet. The location of that information must be the same for all packets forwarded inside the domain.

For packets that are routed by a PCE along a Track, the tuple formed by the IPv6 source address and a local RPLInstanceID in the packet identify uniquely the Track and associated transmit bundle. Additionally, an IP packet that is sent along a Track uses the Differentiated Services Per-Hop-Behavior Group called Deterministic Forwarding, as described in [I-D.svshah-tsvwg-deterministic-forwarding].

For packets that are routed by RPL, that information is the RPLInstanceID which is carried in the RPL Packet Information, as discussed in section 11.2 of [RFC6550], "Loop Avoidance and Detection".

The RPL Packet Information (RPI) is carried in IPv6 packets as a RPL option in the IPv6 Hop-By-Hop Header [RFC6553].

6Lo is currently considering a Next Header Compression (NHC) for the RPI (RPI-NHC). The RPI-NHC is specified in [I-D.thubert-6lo-rpl-nhc], and is the compressed equivalent to the whole HbH header with the RPL option.

An alternative form of compression that integrates the compression on IP-in-IP encapsulation and the Routing Header type 3 [RFC6554] with that of the RPI in a new 6LoWPAN dispatch/header type is concurrently being evaluated as [I-D.thubert-6lo-routing-dispatch].

Either way, the method and format used for encoding the RPLInstanceID is generalized to all 6TiSCH topological Instances, which include both RPL Instances and Tracks.

This architecture operates on IEEE802.15.4 and expects link-layer security to be enabled at all times between connected devices, except for the very first step of the device join process, where a joining device may need some initial, unsecured exchanges so as to obtain its initial key material. Work has already started at the 6TiSCH Security Design Team and an overview of the current state of that work is presented in Section 10.1.

Future work on 6TiSCH security and will examine in deeper detail how to secure transactions end-to-end, and to maintain the security posture of a device over its lifetime. The result of that work will be described in a subsequent volume of this architecture.

a one-hop (radio) neighbor of the joining node that acts as proxy network node and may provide connectivity with the JCE.

The join protocol consists of three major activities:

Device Authentication:

The JN and the JA mutually authenticate each other and establish a shared key, so as to ensure on-going authenticated communications. This may involve a server as a third party.

Authorization:

The JA decides on whether/how to authorize a JN (if denied, this may result in loss of bandwidth). Conversely, the JN decides on whether/how to authorize the network (if denied, it will not join the network). Authorization decisions may involve other nodes in the network.

Configuration/Parameterization:

The JA distributes configuration information to the JN, such as scheduling information, IP address assignment information, and network policies. This may originate from other network devices, for which the JA may act as proxy. This step may also include distribution of information from the JN to the JA and other nodes in the network and, more generally, synchronization of information between these entities.

The device joining process is depicted in Figure 10, where it is assumed that devices have access to certificates and where entities have access to the root CA keys of their communicating parties (initial set-up requirement). Under these assumptions, the authentication step of the device joining process does not require online involvement of a third party. Mutual authentication is performed between the JN and the JA using their certificates, which also results in a shared key between these two entities.

The JA assists the JN in mutual authentication with a remote server node (primarily via provision of a communication path with the server), which also results in a shared (end-to-end) key between those two entities. The server node may be a JCE that arbitrages the network authorization of the JN (where the JA will deny bandwidth if authorization is not successful); it may distribute network-specific configuration parameters (including network-wide keys) to the JN. In its turn, the JN may distribute and synchronize information (including, e.g., network statistics) to the server node and, if so desired, also to the JA. The actual decision of the JN to become part of the network may depend on authorization of the network itself.

The server functionality is a role which may be implemented with one (centralized) or multiple devices (distributed). In either case, mutual authentication is established with each physical server entity with which a role is implemented.

Note that in the above description, the JA does not solely act as a relay node, thereby allowing it to first filter traffic to be relayed based on cryptographic authentication criteria - this provides first-level access control and mitigates certain types of denial-of-service attacks on the network at large.

Depending on more detailed insight in cost/benefit trade-offs, this process might be complemented by a more "relaxed" mechanism, where the JA acts as a relay node only. The final architecture will provide mechanisms to also cover cases where the initial set-up requirements are not met or where some other out-of-sync behavior occurs; it will also suggest some optimizations in case JCE-related information is already available with the JA (via caching of information).

When a device rejoins the network in the same authorization domain, the authorization step could be omitted if the server distributes the authorization state for the device to the JA when the device initially joined the network. However, this generally still requires the exchange of updated configuration information, e.g., related to time schedules and bandwidth allocation.

Special thanks to Tero Kivinen, Jonathan Simon, Giuseppe Piro, Subir Das and Yoshihiro Ohba for their deep contribution to the initial security work, and to Diego Dujovne for starting and leading the On-the-Fly effort.

Special thanks also to Pat Kinney for his support in maintaining the connection active and the design in line with work happening at IEEE802.15.4.

Also special thanks to Ted Lemon who was the INT Area A-D while this specification was developed for his great support and help throughout.

This volume only covers a portion of the total work that is needed to cover the full 6TiSCH architecture. Missing portions include Deterministic Networking with Track Forwarding, Dynamic Scheduling, and Security.